[mod] ci: strict(er) exec (#5099)

All actions are pulled using the version hash, versions are handled by
dependabot, and we'll have control over which actions get updated.

Replaces Trivy scanner with Docker Scout, we have recently begun analyzing the
images there, and the action will keep us in sync about the problems on GHCS
dashboard.

.github/workflows/checker.yml

@@ -24,17 +24,17 @@ jobs:
     runs-on: ubuntu-24.04-arm
     steps:
       - name: Setup Python
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065  # v5.6.0
         with:
           python-version: "${{ env.PYTHON_VERSION }}"
 
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2
         with:
           persist-credentials: "false"
 
       - name: Setup cache Python
-        uses: actions/cache@v4
+        uses: actions/cache@0400d5f644dc74513175e3cd8d07132dd4860809  # v4.2.4
         with:
           key: "python-${{ env.PYTHON_VERSION }}-${{ runner.arch }}-${{ hashFiles('./requirements*.txt') }}"
           restore-keys: "python-${{ env.PYTHON_VERSION }}-${{ runner.arch }}-"